Compressor rotors of gas turbine engines, such as the fan of a turbofan, may experience two main types of aerodynamic instability: stall flutter and supersonic flutter, as shown in FIG. 6. Stall flutter (sometimes simply called “flutter”) is sub-sonic or transonic and may occur when two or more adjacent blades in a blade row vibrate at a frequency close to their natural vibration frequency and the vibration motion between the adjacent blades is substantially in phase. Stall flutter also typically occurs over a limited speed band, often just below design speed conditions.
Supersonic flutter (which can be either stalled or unstalled, as shown in FIG. 6) occurs in the high speed regime of the compressor or fan where tip speed is very high. Unlike stall flutter in the subsonic or transonic flow regime, supersonic flutter can cause an operational barrier—i.e. it is not possible to simply accelerate through a speed range in order to stop and/or limit the effects of supersonic flutter once it occurs. Supersonic flutter may occur under certain flight conditions. Prolonged operation of a fan or compressor rotor undergoing supersonic flutter can produce a potentially undesirable result caused by airfoil stress load levels exceeding threshold values.